[BUG]: Class attributes are not accessible when caught Python exception using py::register_exception

Required prerequisites

• Make sure you've read the documentation. Your issue may be addressed there.
• Search the issue tracker and Discussions to verify that this hasn't already been reported. +1 or comment there if it has.
• Consider asking first in the Gitter chat room or in a Discussion.

What version (or hash if on master) of pybind11 are you using?

2.6.1

Problem description

Hi, I'm encountering an issue when trying to expose custom C++ exceptions to Python using py::register_exception in pybind11. The exception is correctly caught in Python, but the class attributes (e.g., description, errorCode) are not accessible when caught as a Python exception.

Reproducible example code

// Custom C++ exception class with public fields and methods:

namespace Custom {
    class CustomException : public std::exception {
    public:
        std::string description;
        int errorCode;

        CustomException(const std::string& desc, int code)
            : description(desc), errorCode(code) {}

        const char* what() const noexcept override {
            return description.c_str();
        }

        std::string ErrorCodeAsString() const {
            return std::to_string(errorCode);
        }
    };
}

// Bind exception 

void BindCustom(pybind11::module& m) {
    py::register_exception<Custom::CustomException>(m, "CustomExceptionError");

    py::class_<Custom::CustomException>(m, "CustomException")
        .def(py::init<std::string, int>())
        .def("ErrorCodeAsString", &Custom::CustomException::ErrorCodeAsString)
        .def_property("description",
            [](const Custom::CustomException& e) { return e.description; },
            [](Custom::CustomException& e, const std::string& d) { e.description = d; })
        .def_property("errorCode",
            [](const Custom::CustomException& e) { return e.errorCode; },
            [](Custom::CustomException& e, int code) { e.errorCode = code; });

    py::register_exception_translator([](std::exception_ptr p) {
        try {
            if (p) std::rethrow_exception(p);
        } catch (const Custom::CustomException& e) {
            PyErr_SetString(PyExc_RuntimeError, e.what());
        }
    });
}

Is this a regression? Put the last known working version here if it is.

Not a regression

The report names py::register_exception, py::class_, and the exception translator as the relevant entry points, but it does not identify a source file or test. Start by tracing how the registered exception is created and translated, then verify that caught Python exceptions expose the described attributes and methods without breaking the existing reproduction.